An atoll is a kind of island. It is made when a coral reef forms around an island that sinks over many years. In the end, the land is gone, and only the coral reef continues to grow until it becomes an atoll, an island shaped like a doughnut. 
Charles Darwin, who is most famous for his theory of evolution, was the first person to find out how atolls form. He said that volcanoes in the ocean sometimes wear away or sink deeper. Coral growing on a volcano likes to be near the surface, and it keeps growing to stay there.
- Darwin, C. 1842. The structure and distribution of coral reefs, being the first part of the voyage of the Beagle under the command of Capt. Vitzroy R.N. during the years 1832 to 1834. Smith, Elder and Co.: London. 214 pp.